Magneto-optical effect in a system of colloidal particle having anisotropic dielectric property

Spherical, non-cubic crystal nanoparticles with an anisotropic dielectric constant and a weak intrinsic magnetic moment, such as goethite nanoparticles, can be considered to be equivalent to a magnetic dielectric ellipsoid. For colloids based on such ellipsoids, the dielectric constant is isotropic in zero magnetic field. However, when a magnetic field is applied, the moments' Brownian rotation, which is connected with the rotation of the ellipsoids themselves, will induce order in the orientation of the dielectric ellipsoids, so that the dielectric constant of the colloids becomes anisotropic to exhibit magneto-optical effect. As the orientation of the dielectric ellipsoids has been ordered by the magnetic field, the mechanism of optical effects is different from general ferrofluids and liquid crystals, and can be viewed as an optically anisotropic action which results from the coupling of both the magnetic and dielectric properties, i.e. the optical effects only come up when a magnetic field is applied because all the dielectric ellipsoids become aligned. It is confirmed from theory and experiment that for magnetic colloid based on spherical nanoparticles with cubic crystal structure, the magneto-optical effects cannot be due to the reaction of single particles.